A few thoughts. Photojournalism is timely usually. We see pictures that complement stories of today's news. As time passes, many of these pictures begin to have no meaning as the story fades from the public's minds. So pictures of famous people and famous events tend to be the ones that are remembered. When I look at the old photos in the article, there are many I don't know what they represented.
The other issue today is the way they're presented and consumed. Years ago, most people regularly read news magazines and daily newspapers where the photos were published. Today, we tend to get more videos something that we didn't see years ago when there was no TV or internet. You'd catch these news videos in the movie theaters when they had news-shorts-of-the-day. I think we're exposed to less stills than before.
The other thing is that because of cellphones, many of the news photos and videos are from amateurs rather than official photojournalists. That's a huge change.